Please do not force this on users. I hacked the code and recompiled for myself to disable the icon dimming, but the code already has support to variable dimming, all that needs to be implemented is a slider in the panel preferences. It is hard to see what the icons are when they are dimmed so this should be a user selected setting. Windows does not dim minimized icons.
The culprit is here: xfce4-panel-4.8.5/plugins/tasklist/tasklist-widget.c #define DEFAULT_ICON_LUCENCY (50) Let this be variable or either switch from 50 or 100 in panel preferences.
There is a hidden setting to control this (XfceTasklist::minimized-icon-lucency) see http://git.xfce.org/xfce/xfce4-panel/tree/docs/README.gtkrc-2.0
Thanks for pointing out the fix.